The invention relates to a superconducting device having a rotor which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding, whose conductors are arranged in a winding mount which is designed to be thermally conductive, and having a refrigeration unit which has at least one cold head which is thermally coupled to the winding via parts which conduct heat. A corresponding device is disclosed in U.S. Pat. No. 5,482,919.
In addition to metallic superconductor materials such as NbTi or Nb3Sn which have been known for a long time and have very low critical temperatures Tc and which are therefore also referred to as low Tc superconductor materials or LTS materials, metal-oxide superconductor materials with critical temperatures above 77 K have been known since 1987. The latter materials are referred to as high-Tc superconductor materials, or HTS materials, and in principle allow a refrigeration technique using liquid nitrogen (LN2).
Attempts have also been made to create superconducting windings with conductors using such HTS materials. However, it has been found that already known conductors can carry only a comparatively small amount of current in magnetic fields with inductions in the Tesla range. This often means that it is necessary to keep the conductors of windings such as these at a temperature level below 77 K, for example between 10 and 50 K, despite the intrinsically high critical temperatures of the materials being used in order in this way to make it possible to carry significant currents at field strengths of several Tesla. A temperature level such as this is admittedly on the one hand considerably higher than 4.2 K, the boiling temperature of liquid helium (LHe) with which known metallic superconductor elements such as Nb3Sn are cooled. On the other hand, however, cooling with LN2 is uneconomic due to the high conductor losses. Other liquefied gases such as hydrogen with a boiling temperature of 20.4 K or neon with a boiling temperature of 27.1 K cannot be used owing to their danger or their lack of availability.
Refrigeration units in the form of cryogenic coolers with closed helium compressed gas circuits are therefore preferably used for cooling windings with HTS conductors in the temperature range. Cryogenic coolers such as these are, in particular, in the form of the Gifford-McMahon or Stirling type, or are in the form of so-called pulse tube coolers. Refrigeration units such as these also have the advantage that the refrigeration performance is available virtually at the push of a button, and there is no need for the user to handle cryogenic liquids. When refrigeration units such as these are used, a superconducting device such as a magnet coil or a transformer winding is cooled only indirectly by heat conduction to a cold head of a refrigerator (see, for example, “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, J P, 20. -24.05.1996, Verlag Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for the superconducting rotor of an electrical machine which is disclosed in the US-A document mentioned initially. The rotor contains a rotating winding composed of HTS conductors, which can be kept at a desired operating temperature of between 30 and 40 K by a refrigeration unit which is in the form of a Stirling, Gifford-McMahon or pulse tube cooler. In a specific embodiment for this purpose, the refrigeration unit contains a cold head which also rotates, is not described in any more detail in the document, and whose colder side is thermally coupled to the winding indirectly, via elements which conduct heat. Furthermore, the refrigeration unit of the known machine contains a compressor unit which is located outside its rotor and supplies the cold head with the necessary working gas via a rotating coupling, which is not described in any more detail, of a corresponding transfer unit. The coupling also supplies the necessary electrical power via two sliprings to a valve drive, which is integrated in the cold head, of the refrigeration unit. This concept makes it necessary for at least two gas connections to be routed coaxially in the transfer unit and means that it is necessary to provide at least two electrical sliprings. Furthermore, the accessibility to the rotating parts of the refrigeration unit and, in particular, to the valve drive in the rotor of the machine is impeded, since the rotor housing must be open when servicing is necessary. Furthermore, the operation of a known valve drive with fast rotation, as in the case of synchronous motors or generators, is not assured.
Against the background of this related art, one possible object of the present invention is to refine the device having the features mentioned initially such that reliable and economic operation of the refrigeration unit both when at rest and when the rotor is rotating is ensured by it in a temperature range below 77 K, with comparatively reduced hardware complexity.
The superconducting device accordingly has a rotor which is mounted such that it can rotate about a rotation axis and has at least one superconducting winding, whose conductors are arranged in a thermally conductive winding mount, as well as a refrigeration unit which has at least one cold head that is thermally coupled to the winding via parts which conduct heat. In this case, the superconducting device should have the following features, namely                in that the winding mount is equipped with a central, cylindrical cavity which extends in the axial direction and to which a lateral cavity is connected which leads out of the winding mount,        in that the cold head is located in a fixed manner outside the rotor and is thermally connected to a condenser unit for condensation of a refrigerant,        in that a stationary heat pipe is coupled to the condenser unit, which pipe projects axially into the corotating lateral cavity and seals off this area, and        in that the heat pipe, the lateral cavity and the central cavity are filled with coolant, with condensed refrigerant being passed via the heat pipe into the lateral cavity and from there into the central cavity, and refrigerant which is vaporized there being passed back via the lateral cavity and the heat pipe to the condenser unit.        
In consequence, in the refinement of the superconducting device, the entire refrigeration unit is arranged with any moving parts outside the rotor, and is thus easily accessible at any time. The refrigeration power and the heat transfer are provided by a stationary cold head in the rotor via the heat pipe, which ensures that the refrigerant is transported without any mechanically moving parts. In this case, the refrigerant is condensed, with heat being emitted, in a circulating process in a condenser unit, which is connected in a highly thermally conductive manner to the cold head. The liquid condensate then runs through the heat pipe into the lateral cavity and from there into the central cavity in the rotor. The condensate is transported through the heat pipe under the influence of the force of gravity on the basis of a so-called thermosyphon effect, and/or by the capillary force of the inner wall of the heat pipe. In this context, this pipe acts in a manner which is known per se as a “wick”. This function can also be optimized by appropriate refinement or cladding of the inner wall. The condensate drips into the lateral cavity at the end of the heat pipe. This condensate, which is passed from this lateral cavity into the central cavity, which is located in the region of the winding, is at least partially vaporized there. The refrigerant, which is vaporized in this way with heat being absorbed, then flows through the interior of the heat pipe back into the condenser device. The return flow is in this case driven by a slight overpressure in the central cavity, which acts as an evaporator part, relative to the parts of the condenser unit which act as a condenser. This reduced pressure, which is produced by the creation of gas in the evaporator and by the liquefaction in the condenser, leads to the desired refrigerant return flow. Corresponding refrigerant flows are known from so-called heat pipes.